The invention relates to the preparation of a composite of an elastomer composition containing silica-based reinforcement created by a controlled alkoxysilane condensation reaction within the elastomer matrix. The invention also relates to such composite and to a tire having at least one component of such composite.
Elastomer compositions typically contain particulate filler reinforcement such as, for example, carbon black and/or silica.
Silica reinforcement for an elastomer composition may be provided by adding particulate silica such as, for example, precipitated silica, to a rubber composition.
Alternatively, silica reinforcement may be formed in-situ within an elastomer host by a condensation of an alkoxysilane to form, for example, a polysiloxane within the elastomer matrix.
It is a purpose of this invention to enhance such a formation of the alkoxysilane condensation reaction product within an elastomer host.
While it is known that various condensation reactions may be promoted or retarded by the presence of acidic or base materials, such phenomenon as applied to an alkoxysilane, particularly an alkoxysilane polysulfide, condensation within an elastomer host is believed to be novel and inventive.
In practice, particularly where a particulate silica-based filler is added to an elastomer composition as an elastomer reinforcement such as, for example, an aluminosilicate or silica-modified carbon black, a coupling agent is often used to aid in coupling the filler to one or more diene-based elastomers. The use of various coupling agents for such purpose is well known to those having skill in such art.
Such coupling agents may be, for example, an alkoxyorganosilane polysulfide which has a constituent component, or moiety, (the alkoxysilane portion) capable of reacting with, for example, silanol groups, on the silica surface of the silica-containing filler and, also, a constituent component, or moiety, (the polysulfide portion) usually capable of interacting with the rubber, particularly a diene-based, sulfur vulcanizable rubber which contains carbon-to-carbon double bonds, or unsaturation. In this manner, then, the coupler may act as a connecting bridge between the silica-containing filler and the rubber and, thereby, enhance its reinforcing effect for the rubber composition.
In particular, it is envisioned that the silane moiety of the coupler, particularly a trialkoxysilane moiety, is available for reacting with virtually any reactive hydroxyl groups it may encounter and particularly with silanol groups (xe2x80x94Sixe2x80x94OH) on the surface of the silica-based filler and also with alkoxy groups on the coupler itself to form siloxane units (xe2x80x94Sixe2x80x94Oxe2x80x94Sixe2x80x94).
In practice, alkoxyorganosilane polysulfides sometimes used are bis-(3-trialkoxysilylalkyl) polysulfides which contain from about 2 to 8, with an average of from about 3.5 to 4.5, sulfur atoms in its polysulfidic bridge.
During a typical mixing (processing) of the elastomer composition in an internal rubber mixer under high shear conditions to temperatures in a range of 150xc2x0 C. to 175xc2x0 C., the silane portion reacts with the surface of a silica-based filler (e.g.: which may be hydroxyl groups on the surface of the silica which may be in a form of silanol groups).
A portion of the polysulfidic bridge of such organosilane polysulfide typically breaks during such mixing operation, resulting in an exposed sulfur on the coupling agent which is available to apparently interact with one or more of the elastomer(s) in the elastomer composition.
However, it is to be appreciated that such interaction of the sulfur with the elastomer(s) is typically accompanied by an increase in the viscosity (e.g.: Mooney viscosity) of the rubber composition itself as the sulfur interacts with the elastomer(s). Too high of an elastomer viscosity under such conditions makes the rubber composition more difficult to process, or mix, in a typical internal rubber mixer.
Therefore, a degree of adjustment of various formulation ingredients (formulation tuning) and enhancement of various physical properties is limited because of such typical higher viscosity related processing limitations imposed via use of such coupling agent.
Uniquely, however, organosilane polysulfide compounds in a form of organosilane disulfide compounds with predominately contained only about two sulfur atoms in the polysulfidic bridge (e.g. and an average of about 2 to about 2.6 sulfur atoms) do not ordinarily cause such excessive viscosity build-up of the rubber composition during its internal mixing operation.
Such phenomenon in the use of the organosilane disulfide compounds is apparently due to stronger sulfur-to-sulfur bonds for the polysulfidic bridge of the disulfide and their inherent resistance to breaking upon high shear mixing at the aforesaid elevated temperatures, thus, enabling only a very limited amount of sulfur atoms available to interact with diene-based elastomer(s) in the rubber composition during the mixing operation. This phenomenon is well known to those having skill in such art.
Accordingly, it is desired to more effectively utilize the aforesaid rubber processing advantages afforded via use of such organosilane disulfide compound, including a bis-(trialkoxyorganosilane) disulfide, compound as a coupling agent.
The term xe2x80x9cphrxe2x80x9d as used herein, and according to conventional practice, refers to xe2x80x9cparts of a respective material per 100 parts by weight of rubber, or elastomerxe2x80x9d.
In the description of this invention, the terms xe2x80x9crubberxe2x80x9d and xe2x80x9celastomerxe2x80x9d if used herein, may be used interchangeably, unless otherwise prescribed. The terms such as xe2x80x9crubber compositionxe2x80x9d, xe2x80x9ccompounded rubberxe2x80x9d and xe2x80x9crubber compoundxe2x80x9d, if used herein, are used interchangeably to refer to xe2x80x9crubber which has been blended or mixed with various ingredients and materialsxe2x80x9d and xe2x80x9crubber compoundingxe2x80x9d or xe2x80x9ccompoundingxe2x80x9d may be used to refer to xe2x80x9cthe mixing of such materialsxe2x80x9d. Such terms are well known to those having skill in the rubber mixing or rubber compounding art.
In accordance with this invention, a process of preparing a composite of an elastomer composition which contains a silica-based reinforcement therein comprises the sequential steps of:
(A) thermomechanically mixing in at least one preparatory mixing step to a temperature of about 140xc2x0 C. to about 190xc2x0 C., alternatively to about 150xc2x0 C. to about 185xc2x0 C., (i) 100 parts by weight of at least one sulfur vulcanizable elastomer selected from homopolymers and copolymers of conjugated diene hydrocarbons copolymers of at least one conjugated diene hydrocarbon and vinyl aromatic compound, (ii) about 15 to about 100, alternatively about 30 to about 90, phr of particulate filler comprised of at least one silica-based filler which contains reactive hydroxyl groups on the surface thereof (e.g.: silanol groups), (iii) about 0.05 to about 20 parts by weight per part by weight of said particulate filler of at least one organosilane compound, preferably a polysulfide compound and, more preferably, a disulfide selected from at least one of Formula (I) and (II) their mixtures:
Zxe2x80x94R1xe2x80x94Snxe2x80x94R1xe2x80x94Zxe2x80x83xe2x80x83(I) 
Zxe2x80x94R1xe2x80x94Xxe2x80x83xe2x80x83(II) 
wherein n is an integer of from 2 to 8 with an average of from 3.5 to 4.5, or an integer of from 2 to 4 with an average of from 2 to 2.6;
wherein Z is selected from the group consisting of: 
wherein Z is preferably (Z3);
wherein R1 is selected from the group consisting of a substituted or unsubstituted alkylene group having a total of 1 to 18 carbon atoms and a substituted or unsubstituted arylene group having a total of 6 to 12 carbon atoms, preferably alklylenes having from 2 to 6 carbon atoms; wherein R2 may be the same or different and is individually selected from the group consisting of alkyls having 1 to 4 carbons and phenyl; and R3 may be the same or different and is individually selected from the group consisting of alkoxy groups having 1 to 4 carbon atoms, preferably from 1 to 2 carbon atoms; and
wherein X is selected from mercapto, thiocyanato, amino, vinyl, epoxide, acrylate and methacrylate groups, preferably mercapto, epoxide and amine groups; (iv) subsequently mixing therewith at least one silane condensation reaction promoter and/or inhibitor.
(B) subsequently blending sulfur therewith, in a final thermomechanical mixing step at a temperature to about 100xc2x0 C. to about 130xc2x0 C.
For said final mixing step, usually elemental sulfur is added in an amount of about 0.4 to about 3 phr.
In practice, the silica-based fillers are preferably selected from at least one of precipitated silica, aluminosilicate and carbon black which has been treated to contain reactive hydroxyl groups on its surface (e.g.: silanol groups).
In one aspect of the invention such process is provided wherein said preparatory mixing is conducted in at least two sequential thermomechanical mixing steps in at least one internal rubber mixer; wherein at least two of said mixing steps are conducted are conducted to a temperature in a range of about 140xc2x0 C. to about 190xc2x0 C., with intermediate cooling of the rubber composition between at least two of said mixing steps to a temperature below about 50xc2x0 C.
In practice, the total, cumulative period of mixing, namely period of mixing in an internal rubber mixer(s), for the said preparatory mixing step(s) (A) is in a range of about 3 to about 20, alternatively about 5 to about 15, minutes.
In practice, the period of mixing for the final thermomechanical mixing step, namely period of mixing in an internal rubber mixer, (B) is in a range of from about one to about five minutes.
In further accordance with process of this invention, said preparatory mixing step (A) is composed of at least two sequential mixing stages in which said elastomer, said particulate silica-based filler and said organosilane polysulfide compound, preferably an organosilane disulfide compound, are added and mixed in one or more sequential mixing stages and in which said silane condensation reaction promoter or inhibitor is added subsequently to the addition of both of said particulate filler and organosilane polysulfide in the same or subsequent mixing stage.
In additional accordance with the process of this invention, said preparatory step (A) is composed of at least two sequential mixing stages in which about 20 to about 60 weight percent of each of the said silica-based filler, and said organosilane polysulfide compound, preferably an organosilane disulfide compound, are added and mixed in a first preparatory mixing stage and the remainder thereof added and mixed in at least one subsequent preparatory mix stage and the silane condensation promoter or inhibitor added and mixed subsequent to the completed addition of said filler and organosilane polysulfide in the same or subsequent mixing stage.
In practice, the organosilane polysulfide compound, preferably said organosilane disulfide compound, is optionally added to the thermomechanical preparatory mixing step in a form of a particulate comprised of (a) about 25 to about 75, preferably about 40 to about 60, weight percent of said organosilane polysulfide compound and, correspondingly, (b) about 75 to about 25, preferably about 60 to about 40, weight percent carbon black. A purpose of providing the organosilane polysulfide compound in a form of a particulate in the process of this invention is to add the organosilane disulfide in a form of a relatively dry, or substantially dry, powder in which the carbon black acts as a carrier for the organosilane polysulfide in the process of this invention, particularly where the organosilane polysulfide would normally otherwise be in a liquid, or substantially liquid, form. A contemplated benefit for the particulate is to aid in the dispersing of the organosilane polysulfide in the preparatory mixing step(s) of the process of this invention and to aid in the introduction of the organosilane polysulfide into the preparatory mixing of the rubber composition mixture.
In further accordance with this invention, a rubber composition is provided which is prepared by the process of this invention.
In additional accordance with the invention, the process comprises a subsequent step of sulfur vulcanizing the prepared rubber composition, preferably at a temperature in a range of about 140xc2x0 C. to about 190xc2x0 C.
Accordingly, the invention also thereby contemplates a sulfur-vulcanized rubber composition, prepared by the process of the invention.
In additional accordance with the invention, the process comprises the additional, subsequent steps of preparing an assembly of a sulfur-vulcanizable rubber composition having at least one component of an unvulcanized (or vulcanized) rubber composition prepared by the process of this invention and vulcanizing the assembly at a temperature in a range of about 140xc2x0 C. to about 190xc2x0 C.
In one aspect of the invention, such assembly is a tire and, preferably, such component is a tire tread.
Accordingly, the invention also comprises a vulcanized tire prepared by such process.
Also, accordingly, the invention comprises said tire wherein the said component is a tread.
In a further aspect of the invention, optionally a total of about 0.05 to about 5 phr of at least one alkoxy silane, preferably an alkyl alkoxy silane, can be thermomechanically mixed in said preparatory mixing stage(s), particularly where said alkoxy silane has the formula: Rxe2x80x2xe2x80x94Sixe2x80x94(OR)3, where R is a methyl, ethyl, propyl or isopropyl radical, preferably a methyl and/or an ethyl radical and Rxe2x80x2 is a saturated alkyl radical having from 1 to 18, alternatively from 2 to 6, carbon atoms, or an aryl or saturated alkyl substituted aryl radical having from 6 to 12 carbon atoms. Such aryl or substituted aryl radicals might be, for example, benzyl, phenyl, tolyl, methyl tolyl, and alpha-methyl tolyl radicals. Preferably, R is an alkyl radical.
A purpose of the alkoxy silane is, for example, to improve filler incorporation and compound aging. Representative examples of alkyl alkoxy silanes are, for example but not intended to be limited to, propyltriethoxysilane, methyltriethoxy silane, hexadecyltriethoxysilane, and octadecyltriethoxysilane.
For an additional aspect of this invention, optionally from about 5 to about 40 phr of a starch composite of starch and plasticizer may be thermomechanically mixed in said preparatory mixing stage(s).
Such starch composite preferably has a softening point in a range of about 110xc2x0 C. to about 160xc2x0 C. according to ASTM No. D1228.
Starch conventionally has a softening point in a range of about 180xc2x0 C. to about 220xc2x0 C. which is above normal rubber compound mixing temperatures. Accordingly, a starch/plasticizer combination, or as a composite thereof, is used with the plasticizer component having a melting point below 180xc2x0 C. such as, for example, poly(ethylene vinyl acetate), cellulose acetate and diesters of dibasic organic acids, so long as they have a softening point below 180xc2x0 C. and preferably below 160xc2x0 C. It is contemplated that such starch composites contain hydroxyl groups on the surface thereof which are available for reaction with coupling agents as described below. Representative examples of use of starch composites in rubber compositions may be found, for example, in U.S. Pat. No. 5,672,639.
A significant feature of this invention is the utilization of a silane condensation reaction promoter and/or inhibitor to control the rate of in-situ formation, a polysiloxane network which is accomplished by the condensation of the silane coupling agent within the rubber composition.
Accordingly, the silane condensation reaction may be facilitated, for example, by a promoter where it is desired to increase the condensation reaction rate or by a condensation reaction inhibitor where it is desired to retard the condensation reaction rate.
While the mechanism may not be completely understood, it is hypothesized that a polysiloxane-based network (e.g.: a network of at least partially entangled polysiloxane polymer chains) is created by an in-situ formation of a polysiloxane-based polymer chain, resulting from silane condensation, extending into and within already formed polymer chains of the elastomer host. This phenomenon can take place at the interface (filler/elastomer) which means at the vicinity of the filler surface and/or in the polymer matrix.
For example, it is hypothesized that utilization of a bis-(trialkoxyorganosilane) polysulfide compound, preferably a bis-(trialkoxyorganosilane) disulfide compound, in combination with a silane condensation reaction promoter for enhancing a reaction of the bis(trialkoxyorganosilane) polysulfide compound with silanol groups on the silica-based filler and, also, alkoxy moieties of the trialkoxyorganosilane polysulfide compound, react to form a polysiloxane which in turn, may lead to formation of a polysiloxane network at the surface of the filler and/or within the elastomer.
For another significant aspect of the practice of the invention, it is considered herein that a significant departure from past practice, when using a bis-(alkoxyorganosilane) disulfide compound which contains an average of from 2 to 2.6 sulfur atoms in its polysulfidic bridge is (1) promoting a creation of the polysiloxane polymer network via the required promotion of the alkoxysilane condensation reaction which would be accompanied by an expected increase in viscosity of the overall elastomer composition with, correspondingly, a somewhat adverse impact upon its processability because of the increase in rubber composite viscosity yet, however, (2) in the absence of viscosity-increasing sulfur moieties being liberated because the bis-(alkoxyorganosilane) polysulfide compound is a disulfide compound containing the aforesaid average of only 2 to 2.6, instead of the aforesaid 3.5 to 4.5, connecting sulfur atoms in its polysulfide bridge.
Indeed, it is the very use of a bis(alkoxyorganosilane) disulfide compound with the aforesaid sulfur units of the polysulfide bridge being limited to an average of 2 to 2.6, instead of using a bis-(alkoxyorganosilane) polysulfide with an average of 3.5 to 4.5 sulfur atoms in its polysulfidic bridge that enables an operational freedom in processing the unvulcanized rubber composition to be able to tolerate a increase in viscosity of the rubber composition via promoting the aforesaid silane condensation reaction.
As a result, a new method has been discovered to modify, or tune, various physical properties of elastomers via use of chemical control in addition to the thermomechanical mixing conditions.
In the practice of this invention, various acidic or basic condensation promoters may be used and, in general, are understood to be well known to those having skill in such art.
For example, representative of basic promoters are, for example, ammonia, ammonium hydroxide, n-butylamine, terbutylamine, tetrahydrofuran (THF), sodium fluoride, various polyamines such as, for example, pentaethylene hexamine, diaminopropane, diethylenetriamine, triethylenetetramine and polyallylamines such as, for example, poly(allylaminehydrochloride), poly(L-lysine hydrobromide), poly(L-arginine hydrochloride) and poly(L-histidine hydrochloride).
Usually, preferred basic condensation reaction promoters may be amines such as, for example, n-butylamine, or polyamines such as, for example, pentaethylenehexamine and polyallylamines such as, for example, poly(L-lisine hydrobromide, where a basic condensation reaction promoter is desired.
For example, representative of acidic condensation reaction promoters are phosphoric acid, acetic acid, hydrofluoric acid and sulfuric acid; wherein acetic acid and sulfuric acid are usually preferred where an acidic condensation reaction promoter is desired.
Metal salts and metal oxides can also be used as promoters or inhibitors of silane condensation reactions (i.e.: Lewis acid or base reactions).
Representative examples of metal salts are, for example, zinc sulfate, aluminate sulfate, zinc stearate, and aluminum stearate, wherein aluminum stearate may be preferred where a metal salt condensation reaction promoter is desired.
Representative examples of metal oxides are, for example, zinc oxide and aluminum oxide, wherein zinc oxide may be preferred where a metal oxide condensation reaction inhibitor is desired.
Typical catalysts for condensation reaction curing of silicon rubber might also be used. Examples are metal carboxylates such as, for example, bis(2-ethylhexanoate) tin, bis(neodecanoate) tin, and dibutyl tin diacetate, as well as dibutyl tin dilaurate, stannous octoate, stannous acetate, lead naphthanate, cobalt naphthanate and zinc octoate, wherein dibutyl tin dilaurate and cobalt naphthanate may be preferred where a catalyst is desired for use in a silane condensation reaction.
The actual selection of condensation promoter will depend somewhat upon the sensitivity of the rubber matrix, or composition, itself and the balance of desired physical properties. Such selection can readily be made by one having skill in such art.
Thus, the condensation reaction may be controlled by an acid or a base promoter, depending somewhat upon the magnitude of the effect desired and the silica-based filler used. For example, for silica which is acidic in nature, a base type of silane condensation would be expected to be used. For a carbon black type of filler which is basic in nature, an acid type of silane condensation reaction promoter would be expected to be used followed with a base type of silane condensation reaction promoter. It is believed that such use acid/base type of condensation reaction promoters is well known to one having skill in such art.
Thus, for example, while individual circumstances may vary, an acid or base condensation reaction promoter, or any other suitable condensation reaction promoter, may be added to promote, first, the alkoxy silane hydrolysis (acidic promoter) and then, secondly, the silane condensation reaction (basic promoter) leading to a formation of a condensed polysiloxane polymer network within the host elastomer composition.
A typical characteristic of an acid-oriented type of promoter is its possible coordination on the oxygen of the alkoxy group to promote its departure and silane hydrolysis which, in turn, promotes the silane condensation reaction.
A typical characteristic of a base-oriented type of promoter is its possible coordination on the silicon atom of an alkoxyorganosilane to promote a departure of one or more of the alkoxy groups and therefor favors condensation reactions.
Also, either of the acid-oriented type or base-oriented type of promoter may act to disturb the H+OHxe2x88x92 equilibrium at the filler surface thereby indirectly affecting silane condensation also catalyzed by H+OHxe2x80x94. Such phenomena is understood to be well known to those having skill in such art.
According to this invention, a bis-(alkoxyorganosilane) polysulfide, preferably a relatively high purity disulfide material version which does not appreciably release free sulfur under the prescribed conditions, is used in addition to a silane condensation promoter.
While the mechanism might not be completely understood, it is contemplated that a combination of the bis-(alkoxyorganosilane) polysulfide, preferably a disulfide, compound and condensation promoter or inhibitor (retarder) acts to chemically tune, or control, the silane reactivity, assist in the formation of polysiloxane polymer network within the elastomer host with a result of modifying both the processing of the rubber composition and, also, resultant physical properties of the rubber composition.
Where it is desired for the rubber composition, which contains both a siliceous filler such as silica, alumina and/or aluminosilicates, carbon black and silica-modified carbon black reinforcing pigments, to be primarily reinforced with silica-based filler(s) as the reinforcing filler, it is often preferable that the weight ratio of such silica-based fillers to carbon black is at least 1/10 and preferably at least 10/1.
In one aspect of the invention, it is preferred that the silica-based filler is precipitated silica.
In another aspect of the invention, the filler is comprised of about 15 to about 95 weight percent precipitated silica, alumina, aluminosilicate and/or silica-modified carbon black, correspondingly, about 5 to about 85 weight percent carbon black; wherein the said carbon black has a CTAB value in a range of about 80 to about 150.
In a practice of this invention, said filler can be comprised of about 60 to about 95 weight percent of said silica, alumina, aluminosilicate and/or silica-modified carbon black, and, correspondingly, about 40 to about 5 weight percent carbon black.
The aluminosilicate can be prepared, for example, by co-precipitation of a sodium silicate and an aluminate. The preparation of aluminosilicates is well known to those having skill in such art.
The silica-modified carbon black may be prepared, for example, by reacting carbon black with a siloxane or by co-fuming carbon black and silica an elevated temperature. Such methods of modifying carbon black with silica are known to those having skill in such art.
Representative examples of bis-(alkoxyorganosilane) disulfides of the aforesaid bis-(alkoxyorganosilane) polysulfide compound of Formula (I) include, for example:
2,2xe2x80x2-bis(trimethoxysilylethyl) disulfide; 3,3xe2x80x2-bis(trimethoxysilylpropyl) disulfide; 3,3xe2x80x2-bis(triethoxysilylpropyl) disulfide; 2,2xe2x80x2-bis(triethoxysilylethyl) disulfide; 3,3xe2x80x2-bis(triethoxysilylpropyl) disulfide; 2,2xe2x80x2-bis(tripropoxysilylethyl) disulfide; 2,2xe2x80x2-bis(tri-sec.butoxysilylethyl) disulfide; 3,3xe2x80x2-bis(tri-t-butoxysilylethyl) disulfide; 3,3xe2x80x2-bis(triethoxysilylethyl tolylene) disulfide; 3,3xe2x80x2-bis(trimethoxysilylethyl tolylene) disulfide; 3,3xe2x80x2-bis (triisopropoxysilylpropyl) disulfide; 3,3xe2x80x2-bis (trioctoxysilylpropyl) disulfide; 2,2xe2x80x2-bis (2xe2x80x2-ethylhexoxysilylethyl) disulfide; 2,2xe2x80x2-bis (dimethoxy ethoxysilylethyl) disulfide; 3,3xe2x80x2-bis(methoxyethoxypropoxysilylpropyl) disulfide; 3,3xe2x80x2-bis(methoxy dimethylsilylpropyl) disulfide; 3,3xe2x80x2-bis (cyclohexoxy dimethylsilylpropyl) disulfide; 4,4xe2x80x2-bis(trimethoxysilylbutyl) disulfide; 3,3xe2x80x2-bis(trimethoxy silyl-3-methylpropyl) disulfide; 3,3xe2x80x2-bis (tripropoxysilyl-3-methylpropyl) disulfide; 3,3xe2x80x2-bis (dimethoxy methylsilyl-3-ethylpropyl) disulfide; 3,3xe2x80x2-bis(trimethoxysilyl-2-methylpropyl) disulfide; 3,3xe2x80x2-bis (dimethoxyphenylsilyl-2-methylpropyl) disulfide; 3,3xe2x80x2-bis (trimethoxysilylcyclohexyl) disulfide; 12,12xe2x80x2-bis (trimethoxysilyldodecyl) disulfide; 12,12xe2x80x2-bis (triethoxysilyldodecyl) disulfide; 18,18xe2x80x2-bis (trimethoxysilyloctadecyl) disulfide; 18,18xe2x80x2-bis (methoxydimethylsilyloctadecyl) disulfide; 2,2-xe2x80x2-bis (trimethoxysilyl-2-methylethyl) disulfide; 2,2xe2x80x2-bis (triethoxysilyl-2-methylethyl) disulfide; 2,2xe2x80x2-bis (tripropoxysilyl-2-methylethyl) disulfide; and 2,2xe2x80x2-bis(trioctoxysilyl-2-methylethyl) disulfide.
Representative examples of compounds according to Formula (II) are, for example, mercapto propyl triethoxysilane, gamma-amino propyl triethoxysilane, vinyl triethoxysilane, gamma-glycidoxy propyltriethoxy silane, gamma-methacryloxypropyltriethoxysilane.
In general, preferred compounds according to Formula (II) are mercaptopropyltriethoxysilane, gamma-glycidoxy propyltriethoxysilane, gamma-aminopropyltriethoxysilane.
In the practice of this invention, as hereinbefore pointed out, the rubber composition is comprised of at least one diene-based elastomer, or rubber. Suitable conjugated dienes are isoprene and 1,3-butadiene and suitable vinyl aromatic compounds are styrene and alpha-methylstyrene. Thus, it is considered that the elastomer is a sulfur-curable elastomer. Such diene-based elastomer, or rubber, may be selected, for example, from at least one of cis 1,4-polyisoprene rubber (natural and/or synthetic), and preferably natural rubber), emulsion polymerization prepared styrene/butadiene copolymer rubber, organic solution polymerization prepared styrene/butadiene rubber, 3,4-polyisoprene rubber, isoprene/butadiene rubber, styrene/isoprene/butadiene terpolymer rubbers, cis 1,4-polybutadiene, high vinyl polybutadiene rubber (35-90 percent vinyl), styrene/isoprene copolymers, emulsion polymerization prepared styrene/butadiene/acrylonitrile terpolymer rubber and butadiene/acrylonitrile copolymer rubber.
By emulsion polymerization prepared E-SBR, it is meant that styrene and 1,3-butadiene are copolymerized as an aqueous emulsion. Such are well known to those skilled in such art. The bound styrene content can vary, for example, from about 5 to 55%. In one aspect, the E-SBR may also contain acrylonitrile to form a terpolymer rubber, as E-SNBR, in amounts, for example, of about 2 to about 30 weight percent bound acrylonitrile in the terpolymer.
The solution polymerization prepared SBR (S-SBR) typically has a bound styrene content in a range of about 5 to about 55, preferably about 9 to about 36, percent. The S-SBR can be conveniently prepared, for example, by organo lithium catalyzation in the presence of an organic hydrocarbon solvent.
The 3,4-polyisoprene rubber (3,4-PI) is considered beneficial for a purpose of enhancing the tire""s traction when it is used in a tire tread composition. When used, it is usually used in a minor amount of the rubber component(s) of a rubber composition (e.g.: 5 to 15 phr).
The 3,4-polyisoprene elastomer and use thereof is more fully described in U.S. Pat. No. 5,087,668 which is incorporated herein by reference.
The cis 1,4-polybutadiene rubber is considered to be beneficial for a purpose of enhancing the tire tread""s wear, or treadwear.
Such polybutadiene elastomer can be prepared, for example, by organic solution polymerization of 1,3-butadiene as is well known to those having skill in such art.
The cis 1,4-polybutadiene elastomer may be conveniently characterized, for example, by having at least a 90% cis 1,4-content.
The cis 1,4-polyisoprene and cis 1,4-polyisoprene natural rubber are well known to those having skill in the rubber art.
In the practice of this invention, it is further contemplated that alkoxysilane-terminated and tin coupled solution polymerization prepared elastomers may also be used.
The alkoxysilane-terminated elastomers may be prepared, for example, by introduction of a chloro-alkoxysilane, chloro-alkylalkoxysilane or 3,3xe2x80x2-bis-(triethoxysilylpropyl) disulfide, into the polymerization system during the preparation of the elastomer, usually at or near the end of the polymerization. Such termination of such elastomers and the preparation thereof are well known to those having skill in such art.
Tin coupled elastomers are prepared by introducing a tin coupling agent during the polymerization reaction, usually at or near the end of the polymerization. Tin coupling of elastomers is well known to those having skill in such art.
Representative of tin coupled diene-based elastomers are, for example styrene/butadiene copolymers, isoprene/butadiene copolymers and styrene/isoprene/butadiene terpolymers.
In one aspect, it is preferred that a major portion, preferably at least about 50 percent, and more generally in a range of about 60 to about 85 percent of the Sn bonds in the tin coupled elastomer, are bonded to diene units of the styrene/diene copolymer, or diene/diene copolymer as the case may be, which might be referred to herein as xe2x80x9cSn-dienyl bondsxe2x80x9d (or Si-dienyl bonds), such as, for example, butadienyl bonds in the case of butadiene being terminus with the tin.
Creation of tin-dienyl bonds can be accomplished in a number of ways such as, for example, sequential addition of butadiene to the copolymerization system or use of modifiers to alter the styrene and/or butadiene and/or isoprene reactivity ratios for the copolymerization. It is believed that such techniques, whether used with a batch or continuous copolymerization system, is well known to those having skill in such art.
The tin coupling of the elastomer can be accomplished by relatively conventional means and is believed to be well known to those skilled in such art.
Various tin compounds can be used for such purpose and tin tetrachloride is usually preferred.
The tin coupled copolymer elastomer can also be tin coupled with an organo tin compound such as, for example, alkyl tin trichloride, dialkyl tin dichloride and trialkyl tin monochloride, yielding variants of a tin coupled copolymer with the trialkyl tin monochloride yielding simply a tin terminated copolymer.
Examples of tin modified, or coupled, styrene/butadiene might be found in, for example, U.S. Pat. No. 5,064,910.
The vulcanized rubber composition should contain a sufficient amount of silica, and carbon black if used, reinforcing filler(s) to contribute a reasonably high modulus and high resistance to tear. The combined weight of the silica, alumina, aluminosilicates and/or silica-modified carbon black, as well as carbon black, as hereinbefore referenced, may be as low as about 30 parts per 100 parts rubber, but is more preferably from about 35 to about 90 parts by weight.
While it is considered herein that commonly-employed siliceous pigments used in rubber compounding applications might be used as the silica in this invention, including pyrogenic and precipitated siliceous pigments (silica) alumina, aluminosilicates, precipitated silicas are preferred.
The siliceous pigments preferably employed in this invention are precipitated silicas such as, for example, those obtained by the acidification of a soluble silicate, e.g., sodium silicate. Such precipitated silicas are well known to those having skill in such art.
Such precipitated silicas might be characterized, for example, by having a BET surface area, as measured using nitrogen gas, preferably in the range of about 40 to about 600, and more usually in a range of about 50 to about 300 square meters per gram. The BET method of measuring surface area is described in the Journal of the American Chemical Society, Volume 60, page 304 (1930).
The silica may also be typically characterized by having a dibutylphthalate (DBP) value in a range of about 100 to about 350, and more usually about 150 to about 300 cm3/100 g (ASTM D2414).
Further, the silica, as well as the aforesaid alumina and aluminosilicate may be expected to have a CTAB surface area in a range of about 100 to about 220 m2/g (ASTM D3849). The CTAB surface area is the external surface area as evaluated by cetyl trimethylammonium bromide with a pH of 9. The method is described in ASTM D 3849 for set up and evaluation. The CTAB surface area is a well known means for characterization of silica.
Mercury surface area/porosity is the specific surface area determined by Mercury porosimetry. For such technique, mercury is penetrated into the pores of the sample after a thermal treatment to remove volatiles. Set up conditions may be suitably described as using a 100 mg sample; removing volatiles during 2 hours at 105xc2x0 C. and ambient atmospheric pressure; ambient to 2000 bars pressure measuring range. Such evaluation may be performed according to the method described in Winslow, Shapiro in ASTM bulletin, p.39 (1959) or according to DIN 66133. For such an evaluation, a CARLO-ERBA Porosimeter 2000 might be used.
The average mercury porosity specific surface area for the silica should be in a range of about 100 to 300 m2/g.
A suitable pore size distribution for the silica, alumina and aluminosilicate according to such mercury porosity evaluation, is considered herein to be:
five percent or less of its pores have a diameter of less than about 10 nm; 60 to 90 percent of its pores have a diameter of about 10 to about 100 nm; 10 to 30 percent of its pores have a diameter of about 100 to about 1000 nm; and 5 to 20 percent of its pores have a diameter of greater than about 1000 nm.
The silica might be expected to have an average ultimate particle size, for example, in the range of 0.01 to 0.05 micron as determined by the electron microscope, although the silica particles may be even smaller, or possibly larger, in size.
Various commercially available silicas may be considered for use in this invention such as, only for example herein, and without limitation, silicas commercially available from PPG Industries under the Hi-Sil trademark with designations Hi-Sil 210, 243, etc; silicas available from Rhone-Poulenc with, for example, designation of Zeosil 1165MP, silicas available from Degussa AG with, for example, designations VN2 and VN3, Ultrasil 7000GR, etc and silicas commercially available from Huber having, for example, a designation of Hubersil 8745.
Representative examples of alumina for the purposes of this invention are natural and synthetic aluminum oxide (Al2O3). Such alumina can be suitably, synthetically prepared, for example, by controlled precipitation of aluminum hydroxide. For example, neutral, acidic, and basic Al2O3 can be obtained from the Aldrich Chemical Company. In the practice of this invention, the neutral alumina is preferred, however, it is considered herein that the acidic, basic and neutral forms of alumina could be used. The neutral, or substantially neutral form is indicated as being preferential in order to use a form with reduced number of surface xe2x80x94OH groups as compared to the acidic form and, also, to reduce the basic sites of the alumina which are AlO-ions, representing a strong base, in order to reduce potential interferences with the desired reactions of the alumina with the organosilane disulfide coupler.
Representative examples of aluminosilicates for the purposes of this invention are, for example but not intended to be limited to, Sepiolite as a natural aluminosilicate which might be obtained as PANSIL from Tolsa S. A., Toledo, Spain and SILTEG as a synthetic aluminosilicate from Degussa GmbH. Such aluminosilicates can be used as natural materials or synthetically prepared, for example, as hereinbefore exemplified.
It is readily understood by those having skill in the art that the rubber composition would be compounded by methods generally known in the rubber compounding art such as mixing the various sulfur-vulcanizable constituent rubbers with various commonly-used additive materials such as, for example, curing aids, such as sulfur, activators, retarders and accelerators, processing additives, such as oils, resins including tackifying resins, silicas, and plasticizers, fillers, pigments, fatty acid, zinc oxide, waxes, antioxidants and antiozonants, peptizing agents and reinforcing materials such as, for example, carbon black. As known to those skilled in the art, depending on the intended use of the sulfur vulcanizable and sulfur-vulcanized material (rubbers), the additives mentioned above are selected and commonly used in conventional amounts.
Typical amounts of reinforcing type carbon blacks(s), for this invention, if used, are hereinbefore set forth. It is to be appreciated that the silica coupler may be used in conjunction with a carbon black, namely, pre-mixed with a carbon black prior to addition to the rubber composition, and such carbon black is to be included in the aforesaid amount of carbon black for the rubber composition formulation. Typical amounts of tackifier resins, if used, comprise about 0.5 to about 10 phr, usually about 1 to about 5 phr. Typical amounts of processing aids comprise about 1 to about 50 phr. Such processing aids can include, for example, aromatic, napthenic, and/or paraffinic processing oils. Typical amounts of antioxidants comprise about 1 to about 5 phr. Representative antioxidants may be, for example, diphenyl-p-phenylenediamine and others such as, for example, those disclosed in The Vanderbilt Rubber Handbook (1978), pages 344-346. Typical amounts of antiozonants comprise about 1 to 5 phr. Typical amounts of fatty acids, if used, which can include stearic acid comprise about 0.5 to about 3 phr. Typical amounts of zinc oxide comprise about 2 to about 5 phr. Typical amounts of waxes comprise about 1 to about 5 phr. Often microcrystalline waxes are used. Typical amounts of peptizers comprise about 0.1 to about 1 phr. Typical peptizers may be, for example, pentachlorothiophenol and dibenzamidodiphenyl disulfide.
The vulcanization is conducted in the presence of a sulfur-vulcanizing agent. Examples of suitable sulfur-vulcanizing agents include, for example, elemental sulfur (free sulfur) or sulfur donating vulcanizing agents, for example, an amine disulfide, polymeric polysulfide or sulfur olefin adducts which are conventionally added in the final, productive, rubber composition mixing step. Preferably, in most cases, the sulfur-vulcanizing agent is elemental sulfur. As known to those skilled in the art, sulfur-vulcanizing agents are used, or added in the productive mixing stage, in an amount ranging from about 0.4 to about 3 phr, or even, in some circumstances, up to about 8 phr, with a range of from about 1.5 to about 2.5, sometimes from 2 to 2.5, being usually preferred.
Accelerators are used to control the time and/or temperature required for vulcanization and to improve the properties of the vulcanizate. In one embodiment, a single accelerator system may be used, i.e., primary accelerator. Conventionally and preferably, a primary accelerator(s) is used in total amounts ranging from about 0.5 to about 4, preferably about 0.8 to about 1.5, phr. In another embodiment, combinations of a primary and a secondary accelerator might be used with the secondary accelerator being used in smaller amounts (of about 0.05 to about 3 phr) in order to activate and to improve the properties of the vulcanizate. Combinations of these accelerators might be expected to produce a synergistic effect on the final properties and are somewhat better than those produced by use of either accelerator alone. In addition, delayed action accelerators may be used which are not affected by normal processing temperatures but produce a satisfactory cure at ordinary vulcanization temperatures. Vulcanization retarders might also be used. Suitable types of accelerators that may be used in the present invention are amines, disulfides, guanidines, thioureas, thiazoles, thiurams, sulfenamides, dithiocarbamates and xanthates. Preferably, the primary accelerator is a sulfenamide. If a second accelerator is used, the secondary accelerator is preferably a guanidine, dithiocarbamate or thiuram compound.
The rubber composition of this invention can be used for various purposes. For example, it can be used for various tire compounds. Such tires can be built, shaped, molded and cured by various methods which are known and will be readily apparent to those having skill in such art.
The invention may be better understood by reference to the following examples in which the parts and percentages are by weight unless otherwise indicated.